Martensitic stainless steel pipes have been used considerably in recent years in various application uses that require strength and corrosion resistance, particularly, as oil countries tubular goods for petroleum and natural gas wells. With the expansion of applied field, corrosive environments to which steel materials for petroleum and natural gas production are exposed have become more severe. For instance, pressure in the working environments has increased along with the increase of well depth and, in addition, wells have been set increasingly in hostile environments, for example, containing wet carbon dioxide, hydrogen sulfide and chlorine ions at high concentrations. In view of the above, the demand for higher strength has increased and corrosion and embrittlement of tubular goods for oil and gas wells by corrosive ingredients have resulted in a significant problem. Consequently, requirement for higher strength tubular goods with an excellent corrosion resistance has been increased. In the subsequent explanation, "excellent corrosion resistance" means resistance both to "corrosion" and "embrittlement" caused by corrosive ingredients. The embrittlement caused by corrosive ingredient means, for example, sulfide stress corrosion cracking, due to hydrogen sulfide. In the succeeding explanation, "martensitic stainless steel" means both steels in which a martensitic phase after cooling and a transformation constitute a main phase, and steels in which the austenite phase constitutes a main phase at the elevated temperature.
The martensitic stainless steel pipe does not have sufficient resistance to corrosion by sulfide stress corrosion cracking but has excellent resistance to corrosion by wet carbon dioxide. Accordingly, they have been used generally in such environments, that contain wet carbon dioxide at a relatively low temperature. As a typical example, the oil countries tubular goods made of martensitic stainless steels of L80 grade defined by API (American Petroleum Institute) can be mentioned. That is the oil countries tubular goods made of martensitic stainless steels comprising, on the weight percent basis, C: 0.15-0.22%, Si: below 1.00%, Mn: 0.25-1.00%, Cr: 12.0-14.0%, P: below 0.020%, S: below 0.010%, Ni: below 0.50% and Cu: below 0.25%. The L80 grade oil countries tubular goods are generally used mainly in such an environment as containing wet carbon dioxide at a relatively low temperature under a partial pressure of hydrogen sulfide of 0.002 atm or less.
The martensitic stainless steel pipes, including the L80 grade pipes defined by API, generally serve for use after applying hardening and tempering. However, since the start temperature of the martensite transformation of the martensitic stainless steel (it is hereinafter referred to as a Ms point and the finish temperature of the martensitic transformation is referred to as a Mf point) is about 300.degree. C. Such Ms point of martensitic stainless steels is lower compared with that of low alloy steels and the their hardenability is large, so they are highly sensitive to quench cracking. Especially, in the hardening of steel pipes, differing from the case of sheet or rod materials, since high stresses are distributed in a complicated manner, quench cracking is often caused by usual water quenching. Therefore, it was necessary for the hardening of the martensitic stainless steel pipe to adopt a cooling method with a low cooling rate such as intensive air cooling or blast air cooling in order to avoid quench cracking. However, although the above-mentioned method can prevent quench cracking, it involves a problem of poor productivity and the deterioration of mechanical properties and corrosion resistance occur due to the low cooling rate of such method. In the succeeding explanations, "cooling" means "cooling for quenching or hardening", unless otherwise specified.
Generally, the following factors are known for the effects of the cooling rate on the corrosion resistance and the other properties of the martensitic stainless steel pipe.
(a) The sensitivity to sulfide stress corrosion cracking increases as the tensile strength is higher and does not depend on the yield strength. This means that improved strength can be attained without degrading the corrosion resistance by raising the yield strength without increasing the tensile strength of oil countries tubular goods designed for the stress based on the yield strength. Accordingly, in the martensitic stainless steel pipe, increasing the yield ratio(yield strength/tensile strength) is used as an index for judging the performance. It is judged more advantageous as the yield ratio is higher. PA0 (b) Austenite tends to remain in the martensitic stainless steel even after cooling. The residual austenite is decomposed by tempering into ferrite and carbide to lower the yield ratio and the corrosion resistance. PA0 (c) For reducing the residual austenite, the cooling rate has to be increased significantly. It must be much greater than the cooling rate achieved by the air cooling process which is at present adopted. However, blast air cooling or oil quench can not provide a cooling rate, capable of reducing the residual austenite to a level causing no problems. PA0 A method of cooling a steel pipe while rotating a steel pipe around the axis of the pipe axis while making the cooling rate in an entire temperature region at the inner surface of the steel pipe substantially equal to or lower than that at the outer surface of the steel pipe, wherein the cooling rate at the minimum cooling rate position is 8.degree. C./s or higher in a temperature region from "the central temperature between the Ms point and the Mf point" to the Mf point. PA0 (1) A method for cooling a steel pipe, while rotating the pipe around the axis of the pipe, flowing down or spraying cooling water on to the outer surface of a steel pipe, passing cooling water to the inside of the pipe such that the cooling water has a wetting angle of no more than 220.degree., making the cooling rate at the inner surface substantially equal to that at the outer surface and controlling the maximum cooling rate at the inner and the outer surfaces of the steel pipe being 35.degree. C./s or lower thereby cooling the martensitic stainless steel pipe (hereinafter referred to as the "invention [1]"). PA0 (2) A method of cooling a martensitic stainless steel pipe comprising the first cooling of applying air cooling till the temperature at the outer surface of the steel pipe reaches a temperature region from "Ms point -30.degree. C." to "the central temperature between Ms point and Mf point" and the second cooling of successively applying intensive cooling for the outer surface of the pipe at a cooling rate at the inner surface of 8.degree. C./s or higher till the temperature at the outer surface reaches a temperature region lower than Mf point while rotating the steel pipe around the axis of the pipe (hereinafter referred to as "invention [2]"). PA0 (3) A method for cooling a martensitic stainless steel pipe comprising the first cooling by applying intensive cooling to the outer surface till the temperature at the outer surface of the steel pipe reaches a temperature region from "Ms point +400.degree. C." to Ms point, the second cooling of successively applying mild cooling to the outer surface till the temperature at the outer surface reaches a temperature region from Ms point to "the central temperature between Ms point and Mf point", with an average heat transfer coefficient in the second cooling on the outer surface less than 1/2 of that upon completion of the first cooling and the third cooling by applying intensive cooling to the outer surface of the pipe with a cooling rate at the inner surface of 8.degree. C./s or higher till the temperature at the outer surface is lowered below the Mf point, while rotating the steel pipe around the axis of the pipe (hereinafter referred to as the "invention [3]").
A method has been proposed for blowing cooling water by a nozzle to the outer surface of a steel pipe while rotating the pipe and supplying cooling water uniformly over the entire surface of the steel pipe, thereby avoiding uneven cooling (Japanese Patent Laid-Open Hei 3-82711). This method enables cooling to occur at the cooling rate from 1 to 20.degree. C./s, thus more effectively suppressing the residual austenite as compared with existent air cooling. However, the worry of causing quench cracking has not yet been overcome.
Furthermore, as a method of cooling a steel pipe at a high efficiency, there has been a method proposed for supplying cooling water from the end of a steel pipe into the inside, while rotating the pipe and, at the same time, flowing down a laminar cooling water to the outer surface of the steel pipe thereby cooling the inner and the outer surfaces of the steel pipe (Japanese Patent Laid-Open Hei 7-310126). This method can conduct intensive cooling at a cooling rate of 40.degree. C./s or higher and attain efficient cooling. However, the quench cracking has not yet been overcome completely.
Furthermore, an invention relating to a method of cooling a martensitic stainless steel with a specified chemical composition under a specific cooling condition has also been proposed (Japanese Patent Laid-Open Sho 63-149320, Japanese Patent Publication Hei 1-14290, Japanese Patent Laid-Open Hei 2-236257, 2-247360 and 4-224656).
Among them, the Japanese Patent Publication Hei 1-14290 discloses that the sensitivity to stress corrosion cracking is lowered by applying a solution pretreatment to oil countries tubular goods and then cooling at a cooling rate of 1 to 20.degree. C./s. However, quench cracking caused upon rapid cooling is not mentioned at all.
Furthermore, in Japanese Patent Laid-Open Hei 2-236257, Hei 2-247360, Hei 4-224656 and the like, there are provided steels so-called "super 13 Cr" with the C content lower than usual, as well as a manufacturing method for solving both the problems of the corrosion resistance to sulfide stress corrosion cracking and quench cracking. However, since the contents of expensive alloying elements have to be increased in both of the methods, there is a problem of dramatic increase in cost.